Patients will be evaluated periodically for immune function with blood tests, skin tests, and reactions to tetanus, diphtheria, H. influenza B and S. pneumoniae vaccinations. The survival of ADA-corrected cells will be monitored through blood tests. The number and amount of blood tests will depend on the patient s age, weight and health, but is expected that blood will not be drawn more than twice a month. Patients will also undergo bone marrow biopsy aspirate (as described above) twice a year. Patients will be followed once a year indefinitely to evaluate the long-term effects of therapy.